net/http: eliminate defined-but-not-used var.

net/http: eliminate defined-but-not-used var.

gc does not report this as an error, but go/types does.
(I suspect that constructing a closure counts as a reference
to &all in gc's implementation).

This is not a tool bug, since the spec doesn't require
implementations to implement this check, but it does
illustrate that dialect variations are always a nuisance.

LGTM=rsc, bradfitz
R=bradfitz
CC=golang-codereviews, gri, rsc
https://golang.org/cl/73850043